The various stages of oligodendrocyte development and myelination have been described in recent years. However, the specific molecular mechanisms controlling these events are poorly defined. Oligodendrocyte development has been shown to occur in spinal cord explants from embryonic mice on a timetable that recapitulates their development in vivo. We have recently found that cells in the oligodendrocyte lineage fail to develop in spinal cord explants from early embryonic neuregulin knock-out mice. In this grant application we propose to use this model system to address the hypotheses that: 1) neuregulins are necessary at specific developmental stages of the oligodendrocyte lineage, 2) neuregulins are presented to developing oligodendrocyte precursor cells specifically by neurons or astrocytes, 3) the neuregulin receptors erbB2 and erbB4 are required for development of the oligodendrocyte lineage, 4) the late events in oligodendrocyte development, myelination and maintenance of the myelin internode, require the continued presence of neuregulins, and 5) neuregulins activate specific signal transduction pathways in cells of the oligodendrocyte lineage. These hypotheses will be studied using a combination of molecular and cell biological approaches. Neuregulin knock-out mice will be used to determine the specific stages along the oligodendrocyte lineage influenced by neuregulins by adding back exogenous neuregulin to rescue the lineage. Co-cultures of neurons or astrocytes with spinal cord explants will be used to determine if neuregulin is presented to cells in the oligodendrocyte lineage in a cell specific manner. The importance of individual neuregulin receptors in oligodendrocyte development will be studied first in the erbB2 knock-out. Inhibitors of neuregulin will be used to study the role neuregulins play in myelination and maintenance of the myelin internode using both in vitro and in vivo systems. The physical and functional association of activated neuregulin receptors with intracellular signaling molecules will be studied in cells of the oligodendrocyte lineage.